As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. The basis for these techniques is the notion that a sample may be examined by analyzing the reflected energy that results when an optical beam is directed at a sample. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.
One widely used type of optical metrology system includes a pump laser. The pump laser is switched on and off to create an intensity-modulated pump beam. The pump beam is projected against the surface of a sample causing localized periodic excitation of the sample. As the pump laser is modulated, the localized periodic excitation creates a train of thermal and plasma waves within the sample. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
To monitor the surface changes, a probe beam is directed at a portion of the sample that is illuminated by the pump laser. A photodetector records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. For most implementations, this is performed using a heterodyne or lock-in detector (See U.S. Pat. No. 5,978,074 and in particular FIG. 2 for a discussion of such a lock-in amplifier/detector). Devices of this type typically generate separate “in-phase” (I) and “quadrature” (Q) outputs. These outputs are then used to calculate amplitude and phase of the modulated signal using the following equations:Amplitude=√{square root over (I2+Q2)}  (1)Phase=arctan(Q/I)  (2)
The origin of the phase angle and the geometry of “in-phase” and “quadrature” components are illustrated in FIG. 1.
The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for unknown samples can then be analyzed by comparison to the amplitude values obtained for the calibration samples.
Systems of this type (i.e., those using external means to induce thermal or plasma waves in the sample under study) are generally referred to as PMR (photomodulated reflectance) type systems. PMR-type systems are used to study a range of attributes, including material composition and layer thickness. PMR-type systems and their associated uses are described in more detail in U.S. Pat. Nos. 4,634,290, 4,646,088, 4,679,946; 4,854,710, 5,854,719, 5,978,074, 5,074,699 and 6,452,685. Each of these patents is incorporated in this document by reference.
One PMR-type system is manufactured and sold by the assignee herein under the name Therma-Probe and is discussed below with respect to FIG. 8.
Another important use of PMR-type systems is measurement and analysis of the dopants added to semiconductor wafers before and after their activation. Dopants are ions that are implanted to semiconductors during a process known as ion implantation. The duration of the ion implantation process (i.e., total exposure of the semiconductor wafer) controls the resulting dopant concentration. The ion energy used during the implantation process controls the depth of implant. Both concentration and depth are critical factors that determine the overall effectiveness of the ion implantation process.
PMR-type systems are typically used to inspect wafers at the completion of the ion implantation process. The ion implantation damages the crystal lattice as incoming ions come to rest. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective surrogate for direct measurement of dopant concentration and depth.
For this purpose, a PMR-type optical metrology tool with advanced signal processing algorithms is used to record both quadrature (Q) and in-phase (I) components of the signal for a series of specially prepared calibration samples. The measurement method then performs a linear fit using the recorded points to define a calibration line within an I-Q plane. The slope of this line is defined by the implantation energy and the points along the line correspond to different dopant concentrations. Thus, the damage profile can be characterized by comparison of measured and calibration data in I-Q space. Characterization of samples using I and Q outputs is described in U.S. Patent Publication No. 2005/0083528, published Apr. 21, 2005, assigned to the same assignee and incorporated here by reference.
Dopant activation after the ion implantation step is usually performed by rapidly heating and cooling the sample is a special chamber. This process is also known as annealing of semiconductor wafers. During the anneal dopant ions diffuse away from the surface and form a concentration profile within the sample. The transition between the implanted region containing activated dopants and the non-implanted substrate is commonly referred to as a junction. For advanced semiconductor manufacturing, it is generally desirable for the implanted and activated region to be shallow, typically 500 Å or less. Devices of this type are generally referred to as having ultra-shallow junctions or USJ.
A selection of experimentally obtained USJ profiles of six different wafers using secondary ion mass spectroscopy (SIMS) is shown in FIG. 2. We investigated whether it would be possible to characterize an unknown USJ profile (having precisely two unknown characteristics, i.e., junction depth and active dopant level) from a single PMR measurement, involving two independent measurement parameters, i.e., I and Q (or amplitude and phase).
FIG. 3 illustrates schematically two typical USJ profiles having different peak concentration values (C1 and C2, lying on the upper and lower curves respectively). The quality of USJ wafers is usually assessed using three parameters: junction depth (Xj in FIG. 3), peak carrier concentration (C) and junction abruptness (not depicted in FIG. 3). In practice, the required junction depth is often relatively easy for the manufacturer to achieve. However, keeping the USJ profile abrupt and close to the surface with desired carrier concentration is a big challenge. As a result, techniques to measure Xj, C, and abruptness are critical for the manufacture of USJ semiconductors.
A number of techniques have been developed to characterize the effectiveness of USJ process. Destructive and contact methods include secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and spreading resistance depth profiling (SRP). These techniques are capable of providing detailed USJ profile information, but at the expense of a turnaround time that is usually measured in days or even weeks or at the expense of damaging the surface with contacts.
U.S. Patent Publication No. 2004/0251927, published Dec. 16, 2004, assigned to the same assignee and incorporated herein by reference describes a non-destructive PMR-type system for simultaneous measurements of Xj and abruptness. System of this type performs a series of measurements at different separations between the pump and probe beams followed by the analysis of measured data in I-Q space. A similar approach to measuring USJ depth and abruptness is described in the following publications: L. Nicolaides et al., Rev. Sci. Instrum. 74(1), 586 (2003) and A. Salnick et al., Rev. Sci. Instrum. 75(6), 2144 (2004) incorporated here by reference. See also U.S. Patent Publication No. 2005/0253751, published Dec. 16, 2004, incorporated herein by reference.
While USJ depth and abruptness can be characterized using this method in the above cited patent application, no reliable non-contact and non-destructive technique has been described to monitor the peak carrier concentration in USJ samples.